1. Field of the Invention
The field of invention relates to processing hydrocarbon fluids. More specifically, the field relates to an apparatus, method and system for monitoring salts in a hydrocarbon fluid.
2. Description of the Related Art
The amount of salt in a hydrocarbon fluid is a significant factor in several types of transport and refining process problems. Fouling, deactivation of catalyst and severe corrosion are indications of long-term exposure to salt in a feed. The type and amount of salt in the hydrocarbon fluid depends mostly on the source, which in turn can reflect the severity in which hydrocarbons are being drawn from the source or the overall age of the source field. In addition, residual salt water from shipment tankers or handling mishaps can also contaminate the hydrocarbon fluid to a point where the consumer cannot effectively process it. The presence of salt in the hydrocarbon fluid, even as low as in the parts per million (ppm) range, can cause significant damage to refining and chemical processing equipment over a short period.
Dispersed, small water droplets in the hydrocarbon fluid dissolve and retain the salt. The chemical composition of the salt can vary; however, typically the salt found in produced hydrocarbon fluid is sodium chloride with lesser amounts of calcium and magnesium chlorides.
The determination of the salt concentration in the hydrocarbon fluid, especially hydrocarbon fluids made of a crude oil or a natural gas condensate, takes significant amount of effort using current methods. Current testing techniques cannot reliability or with the necessary sensitivity (sub-ppm) determine salt concentration in a timely manner. Laboratory testing and offline measurements are the only reliable means for accurately determining salt concentration in the hydrocarbon fluid to this level. American Standard Test Methods (ASTM) D 3230 & D 6470 are the standardized laboratory tests for measuring the salt concentration in a crude oil. ASTM D 3230 is an electrometric method that requires extensive sample preparation with numerous solvents, including xylene, 1 butanol and methyl alcohol. ASTM D 6470, a potentiometric technique, requires extensive sample preparation as well as additional equipment and labor. Besides being off-line and not expedient, the other procedural and equipment requirements for both of these methods limit their application towards online or “real time” salt concentration determination. As with any testing method, the “human element” can also inadvertently influence the results and add a layer of variability.
Current online salinity sensors depend on directly testing non-conductive paraffinic materials in an attempt to detect the presence of an electroactive specie (that is, salt) in the bulk fluid. Existing sensors use conductive electrodes inserted into flowing crude oil. The salinity sensors apply potential directly into the hydrocarbon fluid in an attempt to determine conductivity of the bulk fluid. This technique sacrifices the testing apparatus by having anodic and cathodic ions directly attack the elements of the apparatus. Corrosion on the sensor probes occurs due to conductive-metal reduction-oxidation reactions on both the working and counter electrodes when water is present. The salts also precipitate directly onto the surface of the probes. Salt on the surface of the probes causes physical deterioration through galvanic pitting even during periods of non-use. Modification to the probe surface area also changes the meaning of surface-area sensitive responses, for example, surface current density or voltage per unit area. The technique is also not accurate because of the general non-conductivity of the bulk hydrocarbon fluid. The presence of heavy paraffin materials can also cause variations in salinity values. As for the method of testing, conductivity can change when a shift in temperature occurs, leading to varying salinity values for the same hydrocarbon fluid sample.
Measuring the salt content in hydrocarbon fluid down to the sub-ppm level before introducing the material to transportation, refining or a desalting process is desirable. Currently, there are no useful techniques to determine salt concentration to the ppm level considered both expeditious and reliable enough to meet commercial transportation and refining needs. A testing apparatus, method and system that makes it possible for expedient salinity determination is desirable. The apparatus and system should be reliable and resilient in the hydrocarbon fluid, especially in use with a salinity treatment system.